1. Field of the invention
The present invention relates generally to the continuous formation of tubes and more specifically to the continuous formation of helically welded pipes or tubes formed from strip material.
2. Description of the prior art
Helically wound pipes may generally be classified by their method of formation as interlocking or welded. In the past, heavy gauge materials were welded to form the helically welded pipe. Non-uniformity of the material strips of lighter gauge steel required that they be interlocked to compensate for the non-uniformity of the edges.
The devices of the prior art have included many complicated mechanisms to either interlock the material or to guide the material into a butt weld. Difficulty has been experienced with the butt-welding of thin gauge metal (generally between 0.020 and 0.030 inches thick) since it cannot be guided by the same mechanisms used for the heavier gauge. The number of parts used in the guiding mechanism of the prior art increases the cost and the reliability of the device.
To assure a perfectly mated edge for butt-welding, prior art devices have overlapped the leading and trailing edges of a piece of strip material and cut the edges that are overlapped prior to welding.
Also it has been considered impossible to continuously weld a seam by MIG or TIG welding using material thinner than 0.030 inches and unthinkable when the material is from 0.008 to 0.014 inches.
One solution provided by the prior art is to provide an interlocking means for thin gauge material and then to subsequently heat the interlocked edge so as to take advantage of the thick and thin material technology. Though providing a sufficient interlock and welded pipe, this device requires precision operation of a group of sub-assemblies to provide the two mating interlocking seams, as well as an alignment relative to the heating element. Another problem faced by the devices used in welding is to provide sufficient tension on the leading and trailing edge so as to guarantee their mating during the welding operation. Complicated belts and rollers have been used to put a twist on the sheet material so as to increase tension and force the butted edges together on the spiral. These systems, as well as others, again increase the number of necessary parts and decrease the reliability of the apparatus of the prior art.